Suspension Tech: How do forks keep air out of their damping circuit?

After last week’s explanation of an IFP (Internal Floating Piston) as the primary method for keeping the oil inside a shock’s damping circuit away from air, we turn our attention to forks. Quick refresher: The oil in your suspension flows through compression and rebound damping circuits, and that’s how the motion is managed so you’re not just bouncing down the trail uncontrollably. It’s the “shock” part of your suspension that controls the “spring”. As oil is pushed through the compression circuit, some of it recirculates behind the piston, but the overflow has to go somewhere.

On rear shocks, there’s usually a reservoir backed by an IFP. In forks, it’s a little different…

The Rockshox SID’s Charger Damper is just one example of a bladder-based method of “pressurizing” the oil to avoid mixing, foaming and, possibly, cavitation.

If, as the oil is pushed through the compression circuit, the oil were to go into an open chamber, there’s a chance it would slosh around, create foam, and small bubbles would get sucked back through the circuit during rebound. Those circuits are finely tuned to perform with a specific oil viscosity. Introduce air or anything else, and the fork will behave much differently. Since air is much less dense than oil, things can get out of control fast. So, most modern forks separate air and oil to create what’s called a “closed cartridge”.

Shown above, Rockshox’s Charger Dampers enclose the damping cartridge system inside a bladder. As oil is pushed through the circuit during compression, it expands the bladder. During rebound, oil is pulled back through and it deflates. The bladder creates a seal between the oil and outside air, preventing them from mixing.

Fox’s FIT dampers have used a closed cartridge with a bladder for several product generations now.

Fox has been doing this with their FIT dampers for years. More recently, X-Fusion (top of post) and DVO have done the same. It’s a lightweight, simple solution that also makes swapping out cartridges easier should they need replacement.

SR Suntour has taken a different approach. On their high end forks with the R2C2 damper, they use a Piston Compensator System. It’s very similar to the IFP in a rear shock in that it uses a piston to separate the air and the oil. The difference is they use a coil spring rather than a nitrogen charge to push back against oil overflow. They say that no matter how well a rubber bladder seals, it’ll eventually need to be bled, and their system makes that easier when the time comes.

Regardless of method, your fork will perform best when air and oil are kept separate, kinda like siblings after a few hours in a car.

Nothing wrong with Open Bath dampers, DVO Emerald, BOS Idylle/Deville and anything fitted with the Avalanche cartridge are fully open bath. You won’t find many complaints about the quality or consistency of damping for those forks. All of Fox’s problems with open bath forks boiled down to a lack of build quality, something that’s been steadily improving since Compass Equity floated the company and got rid of it’s shares.

Also, all production motocross forks were open bath at the begining of this millenium. None are now, but many enduro motorcycle forks are, as having less seals means less stiction and better performance on small chatter.

Technically, keeping the oil under pressure can also help (not totally prevent, just help) with preventing cavitation, but I did update that image caption to make it clearer. I’m working on another article that deals specifically with cavitation, which is largely thanks to all of the comments and feedback from these posts. Should be a good one!

You’re conflating things again. dampers are SEALED & bled to prevent emulsion(oil/air mixing). They’re PRESSURIZED to prevent cavitation(liquid/gas phase change.) part of the reason for the popularity of spring backed compensators(IFPs, bladders, etc,) regardless of whether that spring is coil or air, is that it provides a way to compensate for shaft volume AND pressurize the fluid in one component.

The reason that pressurizing the fluid prevents cavitation is that cavitation is caused by a liquid hitting it’s boiling point at operating temperature due to falling below the pressure that it boils at, at that temperature. When you pressurize the fluid, you’re raising it’s overall pressure, so that when it drops due to vacuum on the back side of piston, it doesn’t drop as low, & therefore does not experience phase change.

In forks, a light spring or just the elasticity of a bladder is usually enough to prevent cavitation, as the pressure drop across their pistons isn’t as much as a that across the pistons in rear shocks.

Tyler, please listen to Groghunter. He’s right about all of this. Cavitation and emulsion are wildly different phenomena. And yes, you’re conflating them: pressurizing the oil helps prevent cavitation, but it’s the seals that prevent emulsion. Really.

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